The invention pertains to a polymer-bonded granular absorptive, adsorptive, chemisorptive, or catalytically active material, which is produced by mixing the absorptive, adsorptive, chemisorptive, or catalytically active fine-grained material with a finely particulate meltable polyethylene with the addition of a binding agent having an oligocondensate basis (or backbone), as well as a process for producing molded bodies from this material.
Molded filter bodies with an adsorptive effect based on activated carbon are known in the prior art. The following substances are used as matrices for the activated carbon:
open-pore foamed plastics
phenoplasts
polyurethanes
plaster of paris
paper substrates
carbon networks (or carbon backbone)
The basic principle underlying inventions of this type is that of introducing activated carbons into the polymeric body or of forming a carbon matrix integral to the activated carbon itself (DE Al 41 40 455).
The solution described in DE C2 37 19 419 uses open-pore foamed plastic as the substrate material.
DE Al 39 25 693 suggests forming a three-dimensional matrix out of the carbon by coating it with a binding agent, but does not elaborate.
A similar solution is proposed in DE Al 38 135 64.
DE Al 41 40 455 describes a process for producing composite adsorbents which are characterized by high abrasion resistance and consist of highly porous inorganic filler materials and a chemically resistant and porous matrix. This is attained by carbonizing the water-soluble binding agent, e.g. preferably pitch acid.
DE Ul 91 15 610 also describes foamed plastics, preferably well foamed polyurethanes, into which the activated carbons are introduced.
According to EP Al 04 92 081, a mixture of cellulose, polyvinyl alcohol, and activated carbon is formed into a hexagonal body with a controlled pore size. The product is well suited for adsorbing aerial impurities, although it does not achieve satisfactory capacities.
DE Al 34 43 900 and DE C3 24 00 827 describe carbon-impregnated textiles and nonwoven fabrics. These solutions use special polymers with a polyurethane basis, e.g. polyurethane fluoride and polytetrafluoroethylene urethane.
The Romanian patent RO 10 40 21 also describes a porous polyurethane support material containing granulated activated carbon in its pores. The granular material is produced by impregnation with a solution of 15 to 20% activated carbon powder and 15% binding agent, which demonstrates that these capacities are not satisfactory either.
Interesting processes for producing molded bodies out of activated carbon for use in gas masks are claimed in U.S. Pat. No. 5,078,132, EP 03 09 277, and WO 94 03 270. These solutions describe a self-supporting porous gas filter material consisting of a molded body containing characteristically defined particles of the adsorptive material and the thermoplastic binding agent. The individual particles are fused into a molded filter body with open pores. It is characteristic of these solutions that the size of the binding particles is less than 20% of the average size of the adsorbent particles. The disadvantages are that satisfactory results with respect to air resistance and capacity are obtained only when thermoplastic polyurethane is used and that relatively high polymer components (ca. 20% of mass) are needed.
The disadvantages of prior art solutions lie in the fact that when the polymer and adsorbent particles are mixed, the phase distribution is always non-uniform for subsequent processing, and this has a strong adverse effect on the quality especially with respect to air resistance and product capacity.
Thermoplastic viscid polyurethanes are known for the technical difficulties associated with treating them and for an enormous cost factor in using them. The polymers generally make up 20% of the product""s mass and melting them results in considerable coverage of the adsorbent surface, which leads to significant losses in capacity and an increase in volume resistance. Moreover, suitable spraying of the thermoplastic polyurethane on the activated product also covers the active surface to a not inconsiderable extent and thereby reduces capacity.
DE Al 195 14 887 describes a solution for producing an adsorptive, pliable, filter surface structure on the basis of a flexible surface structure and polyolefins, among other things, are specified but not elaborated upon for binding the adsorbent particles. On account of the textile substrate material, the solution is not suitable for producing molded bodies of different shapes and is not comparable with the solution of the present invention, which does not need substrate materials. The solution proposed in DE Al 42 38 142 contains porous bodies having adsorptive properties and mentions polyolefins as the binding agent but does not elaborate. The adsorbent particles and binding agent particles which it describes are of comparable sizes, so this does not compare with the solution of the present invention due to different separation problems.
A need has thus been recognized to develop a polymer-bonded granular adsorptive, absorptive, chemisorptive, or catalytically active material which is capable of forming an open-pore and sorptive foamed body at increased temperatures while not reducing the specific surface of the active material, and with a binding effect only during the mixing and processing phases, as well as a process for producing molded bodies from this material.
A need has also been recognized to develop an adsorptive, absorptive, chemisorptive, or catalytically active material and a technological process for producing molded bodies from this material which overcome the disadvantages of the prior art and achieve higher capacity, simpler processing, and minimized costs for at least the same degree of mechanical stability.
The aforementioned needs have been met with an unanticipated solution, in that a thermoplastic polymer with a slight amount of coverage on the surface of the active material was found along with a suitable binding agent by which optimal uniform distribution of the granular thermoplastic polymers in the active material is ensured and the polymer particles are bound to the granular active material for longer periods of processing, while minimizing the wetting of the material""s active surface with the binding agent entering a chemically inert state during the hardening process and thus minimizing reduction in capacity for the active material.
Low-density polyethylene with a low melting point was determined by the invention to be a low-coverage thermoplastic polymer on active materials. The polyethylene enables a strong mechanical binding effect among the particles of the active material, allowing mechanical operations, such as sawing, grinding, drilling, etc., on the finished molded bodies.
The active surface of the active material is only minimally covered ( less than 2% loss of specific surface). A polymer amount as low as 5% by mass of the active material enables sufficient mechanical stability of the molded bodies for certain uses. The problem of efficient uniform distribution of the thermoplastic polymers in the active material over long periods of processing was solved by using a fixing agent (or binding agent) having a modified amino resin oligocondensate basis (or backbone). A melamine resin precondensate partially etherified with methanol and modified and neutralized with triethanolamine is especially advantageous for wetting the thermoplastic polymers with the aminoplast, for binding to the active material while minimally wetting it, and for inert behavior following the thermal process. During the thermal treatment, the amino resin precondensate cross-links to a polymethylene melamine while forming an open-pore, foam-like polymer with good sorptive properties due to its high specific surface (up to 300 m2/g). These sorptive properties extend to gases, ions, and other chemical species such as oils, solvents, etc.
The fixing or binding agent used thus contributes to the creation of sorptive surfaces. It is important to use binding systems which contain no organic solvents, i.e. to use water-dilutable amino resin precondensates.
If binding systems which contain organic solvents are used, the solvent components are instantaneously adsorbed on the active material, the binding or fixing components become dry, and the above-mentioned separation of components affects all subsequent processing. Water-dilutable systems, however, remain in a viscid, flowable state for at least 120 minutes and thus can be easily processed over this period of time.
Experiments were run, for example, in which low-pressure polyethylene in particulate form (e.g. MW 35,000; MN 7,700) with a melting point between 85 and 140xc2x0 C. was mixed with a melamine resin precondensate in a suitable container with a stirring apparatus. When the polyethylene is thoroughly mixed and wetted with the amino resin precondensate, a granular activated carbon, for example, is added and intensive mixing continues until homogenous. The mixture is then passed into a suitable molding device and formed at low pressures and temperatures of approximately 100xc2x0 C. During this procedure, the amino resin components foam, cross-link, and cure, and at the same time the polyethylene body melts as polymer bridges are formed between the individual grains of activated carbon. When cooled for a brief period of time and released from the mold, the result is a molded body which can undergo mechanical processing.